1. Field of the Invention
This invention relates to a hydrous gel essentially consisting of a cured water-in-oil emulsion in which a liquid rubber is used.
2. Description of the Prior Art
Heretofore hydrous gels comprising water soluble polymers such as polyvinyl alcohol, polyacrylic acid, cellulosic polymers and starch have been used as cold- and heat-retaining materials, perfume-retaining materials and shock-absorbing materials and for other various purposes. However, those hydrous gels that are formed following gelation of aqueous solutions of water soluble polymers properly function only in a very limited temperature range, such as in the neighborhood of ordinary temperatures. At temperatures low enough to freeze water, such a hydrous gel is frozen as a whole and turns into a very hard, rigid mass. Conversely, at high temperatures, water evaporates rapidly unless the hydrous gel is held in a tightly closed container, and the remaining mass no longer functions as a hydrous gel; in some cases flow of the material which may occur at places where the points of crosslinking are scarce or the crosslinking bonds are weak may cause various troubles. In using such a hydrous gel, various problems are encountered due to such troubles. Thus, for example, when used as a cold-retaining material, the hydrous gel is frozen as a whole and thereby loses its original flexibility and gives a bad, unpleasant feeling to the human body when it is cooled therewith and moreover it is difficult to cool efficiently those human body parts that rise and fall in a complicated manner. In cases where the hydrous gel is used as a perfume-retaining material, an increase in temperature results in an increased rate of water evaporation, which in turn inevitably causes an accelerated volatilization of the perfume ingredient; and as a result the perfume performance is lost in a very short period of time. Therefore, in the areas of cold- and heat-retaining materials, perfume-retaining materials, shock-absorbing materials and so forth, a hydrous gel which is constant in its function over a wide temperature range is in great demand. A polymeric material satisfying the requirement that the change in its performance characteristics is small over a wide temperature range can be prepared by crosslinking a rubber-like material having a low glass transition point, and it is expected that the above requirement can be met in principle by a cured product from a water-in-oil dispersion in which water is dispersed in the form of droplets in a continuous phase of the above-mentioned rubber-like material.
In fact, some techniques have been proposed in relation to preparing hydrous gels by crosslinking water-in-oil dispersions in which water is dispersed in the form of droplets in rubber components based on natural rubbers, polybutadiene rubbers, other solid rubbers, ethylene-vinyl acetate copolymers, styrene-butadiene copolymers, styrene-isoprene rubber copolymers or other thermoplastic rubbers. However, each technique has its own problems and moreover, the hydrous gel obtained after crosslinking the dispersion does not always have satisfactory performance characteristics. Thus, when a solid rubber such as a natural rubber is used, its viscosity is so high that water practically cannot be emulsufied therein at temperatures below 100.degree. C. even with the use of a small amount of a plasticizer. The technique consisting in dispersing water in a solution of a solid rubber in an organic solvent alone can be considered only as a suggestion; there has been no successful method proposed to remove the organic solvent alone from the hydrous gel so prepared. In the case of the Akio Iwama et al. invention, U.S. Pat. No. 4,102,807, which uses a thermoplastic rubber, a temperature of 150.degree. C. or above is required at the time of emulsification and dispersion of water unless a plasticizer is used. Even when a large amount of a plasticizer is used, it is still necessary to mix the thermoplasic rubber and the plasticizer in advance at such high temperatures as 120.degree. C. to 150.degree. C., so that the processability is far from satisfactory. Moreover, the hydrous gel finally obtained is inferior in heat durability because the points of crosslinking in the thermoplastic rubber involve physical bonds alone. Thus, when the temperature reaches and exceeds 60.degree. C., the crosslinking points clearly show a tendency toward softening. In addition, since the thermoplastic rubber generally contains in part those copolymer portions that have high glass transition temperatures, the rubber component, when kept at low temperatures, partly hardens even if the whole rubber component is not crystallized, and as a result the hydrous gel as a whole becomes rather hard and rigid.